Iron-binding proteins are carrier proteins and metalloproteins that are important in iron metabolism and the immune response. Iron is required for life. Iron-dependent enzymes catalyze a variety of biochemical reactions and can be divided into three broad classes depending on the structure of their active site: non-heme mono-iron, non-heme diiron , or heme centers. A well-known family of iron-dependent enzymes include oxygenases that facilitate hydroxyl group addition of one or both atoms from o2. Notable enzymes include tryptophan dioxygenase, ferredoxin, and 2-oxoglutarate dioxygenase. Heme proteins Heme proteins are proteins that contain a heme prosthetic group. The heme group consists of a porphyrin ring coordinated with an iron ion. Four nitrogen atoms in the porphyrin ring act as a ligand for the iron in the center. In many cases, the equatorial porphyrin is complemented by one or two axial ligands. An example of this is in hemoglobin, where the porphyrin works together with a histidine side chain and a bound O2 molecule, forming an octahedral complex. Hemoglobin Hemoglobin is an oxygen-transport protein found in virtually all vertebrates. Hemoglobin A is the main type found in human adults. It is a tetramer consisting of two alpha and two beta subunits. Each of the four monomeric units contain a heme prosthetic group in which a ferric cation is bound between four nitrogen atoms of a porphyrin ring. Along with a histidine, the apo form has five ligands surrounding the iron atom. Oxygen binds to the empty sixth position to form an octahedral complex in the holo form. Oxygen binding is fully cooperative for each of the subunits because as the first oxygen binds to one of the four heme groups, the protein undergoes a drastic conformational change that sharply increases the oxygen affinity of the other three subunits. Hemoglobin has various affinities, depending on pH, structure, and CO2 partial pressure. Fetal hemoglobin is a variant containing two gamma subunits instead of two beta subunits. Fetal hemoglobin is the predominant form up until the infant is several months old, and it has a greater oxygen affinity to compensate for the low oxygen tension of supplied maternal blood during pregnancy. Hemoglobin has a lower oxygen affinity at low pH. This allows for rapid dissociation as oxygenated hemoglobin is transported to cells throughout the body. Because of the CO2 production and aqueous formation of carbonic acid in respiring cells, oxygenated hemoglobin dissociates in order to deliver